The present invention relates to a process for making gluconic acid or its alkali metal salts by oxidizing glucose with oxygen or with an oxygen containing gas in an aqueous alkaline solution in the presence of a supported catalyst containing a platinum group metal and bismuth.
Large-scale fermentation is one known method for making gluconic acid; another method is catalytic and is based on glucose which is subjected to oxidation with an oxygen containing gas in an aqueous alkaline solution in the presence of a heterogeneous catalyst.
A representative procedure is described in the European patent disclosure 0 142 725which employed a palladium-bismuth catalyst supported on activated charcoal. This method leads to a glucose conversion of 99.8% and a yield of gluconic acid in the form of its sodium salt of 99.5%. The selectivity for formation of gluconic acid sodium salt is stated being 99.7%. The proportion of the fructose isomerization product is stated as being reduced to 0.1%. However, the catalytic activity value is no higher than 1,450 g of product per gram of Pd used per hour of reaction time. Because of the accuracy of measurement of .+-.2% of the HPLC method or ion-chromatography, the above data concerning the selectivity and the quantity of the fructose isomerization product are questionable.
The German OLS 28 36 327 describes the oxidation of aryl glycols by means of an oxygen containing gas in an aqueous, alkaline medium into the corresponding alphaketocarboxylic acid esters using a platinum-bismuth catalyst on an activated charcoal support.
The European Patent 0 005 779 describes the oxidation of alpha-hydroxyaryl acetic acids into arylglyoxyl acids using an oxygen containing gas in an aqueous, alkaline medium with platinum catalysts in the simultaneous presence of lead and/or bismuth or their compounds. A similar procedure is disclosed in Japanese Kokai Tokkyo Koho JP 56/158733 A 2.
Lastly, the European patent disclosure 0 151 498 discloses the preparation of alpha-ketogluconic acid from glucose with a platinum-bismuth catalyst on activated charcoal, however, the weight ratio of catalyst to glucose is 1.17:1.
In all known systems of reaction employing a platinum-bismuth catalyst on activated charcoal therefore the alpha-hydroxy group is oxidized together with an alcohol group, an aldehyde group or a carbonyl group.
Now it has been found that glucose can be made to react with oxygen in an alkaline medium to form gluconic acid in a selective manner and at a much higher rate of reaction than that of the European patent disclosure 0 142 725, provided that the reaction is carried out on a selected supported catalyst. The latter contains platinum, palladium and bismuth and uses activated charcoal as the support. When employing this catalyst, it is possible to use a quantity of catalyst which is so lowered relative to the amount proposed in the European patent disclosure 0 151 498 that at most a weight ratio of catalyst to glucose of 0.2:1 is required.